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暧昧 is a kind of relationship betweenwomen and men, it exists in friendship, but the degree is higher than friendship, it’s almost love but has not become love yet.

I don’t think there is a perfect English word which can perfectly express this meaning, actually we always can’t find a correct word which has exactly the same meaning in another language, so don’t worry about this.

I'd say its similar to being flirtatious with someone, but it's more Chinese culture related.

Because Chinese culture puts emphasis on being indirect, the relationship between a men and women can be somewhat unclear. 暧昧 describes the relationship of two people between "friendship" and "partnership". Think of two people with mutual affection but haven't told each other their feelings. I know it is also common in western world but I would say it exists more in Chinese couples.